Defrost control for refrigerating apparatus



Dec. 19, 1961 v. G. SHARPE DEFROST CONTROLFOR REFRIGERATING APPARATUS Filed July 20, 1959 Fig.

INVENTOR. Ver/os 6. Sharpe BYZ 8 His A f/om 3,013,400 DEFROST CONTROL FOR REFRIGERATING APPARATUS Verlos G. Sharpe, Xenia, Ohio, assignor to General Motors Corporation, Detroit, Mich., a corporation of Dela- Ware Filed July 20, 1959, Ser. No. 828,265

6 Claims. (Cl. 62--156) This invention relates to refrigerating apparatus and more particularly to an improved arrangement for controlling the defrosting of the evaporator.

It is a well-known fact that the rate at which frost forms on the evaporator of a refrigerator varies from day to day depending upon the ambient wet and dry bulb temperatures, the type of products placed in the refrigerator, the number and length of door openings, and similar factors. Consequentially, conventional defrost control systems operated by a clock or by the number of door openings do not cause defrosting to take place in accordance with actual requirements. 7

It is an object of this invention to provide an improved defrost control arrangement which automatically compensates for variations in the many factors which vary the frequency at which defrosting should take place.

Another object of this invention is to eliminate the need for a timerclock and limiter switch.

A further object of this invention is to provide a defrost control arrangement having improved reliability.

Still another object of this invention is to provide a defrost control arrangement which eliminates objectionable clock noises. r

Further objects and advantages of the present invention will be apparent from the following description, reference being had to the accompanying drawing wherein preferred embodiments of the invention are clearly shown.

In the drawings:

FIGURE 1 is a schematic view showing a preferred embodiment of the invention.

FIGURE 2 is a fragmentary sectional view on an enlarged scale showing the defrost control element.

Referring now to the drawings wherein a preferred embodiment ofthe invention has been shown, reference numeral 10 designates a conventional insulated freezer compartment cabinet having a freezer compartment 12 and a compressor compartment 14 provided therein. The compartment 12 is provided with the usual type of access door 16 and is adapted to be cooled by means of an evaporator 18. The evaporator 18 is connected in series refrigerant flow relationship with a conventional motor compressor unit 20, a condenser 22 and a pressure reducing device 24. A temperature responsive switch 25 cycles the compressor unit so as to maintain the temperature in the compartment 12 within the desired temperature range.

During normal operation of the refrigerating system, frost will accumulate on the evaporator 18 and it therefore becomes necessary to remove periodically this frost. For purposes of illustration there is shown an arrangement in Which this is done by means of an electric heater 26 arranged in thermal exchange relationship with the evaporator 18.

The control for the defrost heater 26 includes a thermistor 28 located in a hermetically sealed glass or similar receptacle 30 disposed adjacent one wall of the refrigerator as best shown in FIGURES l and 2 of the drawings.

The thermistor element 28 is connected in series circuit relationship with a secondary transformer coil 32 and a heating element 34. A primary transformer element 36 connected across the main power line 38 is arranged in inductive relationship to the secondary transformer ele- United States Patent "ice ment 32 whereby current is at all times supplied by the transformer coil 32.

As shown in FIGURE 2 of the drawing, the receptacle 30 is disposed with its upper face arranged in thermal exchange relationship with the interior of the food storage compartment 12. A quantity of liquid such as water 40 is provided in the receptacle 30 and normally submerges the thermistor element 28 so as to maintain it in a relatively cool condition. The lower portion of the receptacle 30 is disposed in heat exchange relationship with the ambient air surrounding the freezer compartment 12 whereby the water 40 in the lower part of the receptacle will not become frozen. During normal operation of the refrigerating system, the water 40 will tend slowly to vaporize and will collect on the upper Wall of the receptacle 30 in the form of frost.

The rate at which the Water 40 evaporates will be determined to a certain degree by the temperature of the ambient air surrounding the lower part of the receptacle 30 and will be determined in part by the temperature in the freezer compartment 12. The thermistor element 28 also assists in heating the water. After the refrigerating system has been operating for a longer length of time, the water 40 will have been evaporated in sufficient quantity so that the thermistor element 28 will no longer be submerged in the water. The rate at which the water vapor from the body of Water 40 freezes on the top wall of the receptacle 30 will depend on the temperature of the top of the receptacle 30 and this temperature is influenced by the frequency and length of door openings and other factors which likewise aifect the frequency at which the evaporator needs defrosting. When most of the water has been vaporized, the thermistor element 28 will tend to heat up rather rapidly with the result that its resistance will rapidly decrease. With such a decrease in the resistance of the thermistor element 28 the heater element 34 will be heated to a high enough degree to cause the bimetallic switch actuating element 42 to open the circuit to the compressor 20 at 44 and to close the circuit to the defrost coil at 46.

Energization of the defrost coil or heater 26 will cause melting of the frost on the evaporator 18 and as this takes place the frost formed on the upper Wall of the receptacle 30 will melt. As this latter frost melts the Water will collect around the thermistor element 28 so as to thereby cool the thermistor to the point where it will reduce the flow of current to the heating element 34. This reduced flow of current to the heating element 34 will cause the bimetallic element 42 to reclose the motor compressor circuit at 44 and will open the circuit to the heater element 26.

For purposes of illustrating the invention, the evaporator 18 has been shown in the upper rear corner of the food compartment 12 whereas it could be located in any other suitable location. Likewise, the heating coil 26 has been shown arranged right beneath the evaporator 18 whereas this coil could be arranged so as to be in contact with a major portion of the evaporator 18 and possibly a portion of the receptacle 30.

The thermistor element 28 is of the type that has a negative temperature resistance characteristic whereby as the temperature of the thermistor element 28 increases above a predetermined value the resistance of the element 28 decreases very materially so as to allow a much greater amount of current to flow through the element 28. The housing 30 can be made of glass or any other suitable material. The liquid 40 is permanently sealed within the housing 30 whereby the quantity of liquid never varies. For purposes of illustration, reference has been made to the liquid 40 as being water Whereas it is within the purview of this invention to utilize any other suitable liquid within the receptacle 30.

While the embodiments of the invention as herein disclosed constitute preferred forms, it is to be understood that other forms might be adopted.

What is claimed is as follows:

1. in combination, a refrigerator cabinet having a food storage compartment therein, an evaporator for cooling said food storage compartment, refrigerant liquefying means for supplying liquid refrigerant to said evaporator, means for defrosting said evaporator, control means for said defrosting means comprising a fluid receptacle having one portion arranged in thermal exchange relationship with said food storage compartment, a thermistor unit disposed in the lower portion of said receptacle at least partially below the maximum liquid level therein, means for connecting said thermistor element to a source of electrical energy, and means responsive to a given change in resistance of said thermistor for defrosting said evaporator.

2. In combination, means forming an insulated food storage compartment, an evaporator for cooling said compartment, a refrigerant liquefying means for supplying liquid refrigerant to said evaporator, means for defrosting said evaporator, control means for said defrosting means comprising a fluid receptacle having its upper portion arranged in thermal exchange relationship with said food storage compartment, 2. thermistor unit disposed in said receptacle, a. quantity of liquid in said receptacle, a heater element, a source of electrical energy, means for connecting said thermistor, said heater element, and said source of electrical energy in series circuit relationship, means responsive to a given change in the resistance of said thermistor for defrosting said evaporator.

3. in combination, means forming an insulated food storage compartment, an evaporator for cooling said compartment, a refrigerant liquefying means for supplying liquid refrigerant to said evaporator, means for defrosting said evaporator, control means for said defrosting means comprising a fluid receptacle having its upper portion arranged in thermal exchange relationship with said food storage compartment, a thermistor unit disposed in said receptacle, a quantity of liquid in said receptacle, a heater element, a source of electrical energy, means for connecting said thermistor, said heater element, and said source of electrical energy in series circuit relationship, means responsive to a given change in the resistance of said thermistor for defrosting said evaporator, the lower portion of said receptacle being disposed in thermal exchange relationship with the air surrounding said cabinet.

4. In combination, means forming an insulated food storage compartment, an evaporator for cooling said compartmcnt, a refrigerant liquefying means for supplying liquid refrigerant to said evaporator, means for defrosting said evaporator, control means for said defrosting means comprising a fluid receptacle having its upper portion arranged in thermal exchange relationship with said food storage compartment, a thermistor unit disposed in said receptacle, 9. quantity of liquid in said receptacle, a heater element, a source of electrical energy, means for connecting said thermistor, said heater element, and said source of electrical energy in series circuit relationship, means responsive to a given change in the resistance of said thermistor for defrosting said evaporator, said means for defrosting said evaporator comprising electric heating means in thermal exchange relationship with said evaporator, means for supplying current to said electric heating means, and bimetallic switch means adjacent said heater element for controlling the flow of current to said electric heating means.

5. In a food freezer, the combination, means forming an insulated cabinet having a frozen food storage compartment therein, an evaporator for cooling said compartment to below freezing temperatures, a compressor, a condenser, refrigerant flow connections between said evaporator, compressor and condenser, an electric defrost heater for defrosting said evaporator, control means for said electric defrost heater comprising a fluid receptacle having one portion arranged in thermal exchange relationship with said food storage compartment, a thermistor unit disposed in said receptacle, a quantity of liquid sealed in said receptacle, a heater element, a source of electrical energy, means for connecting said thermistor, heater element, and said source of electrical energy in series circuit relationship, thermostatically operated switch means for controlling the flow of current to said defrost heater, said thermostatically controlled switch means including a bimetallic element arranged in thermal exchange relationship with said heater element.

6. In combination, a refrigerator cabinet having a food storage compartment, a refrigerating system comprising an evaporator for cooling said food storage compartment, and refrigerant liquefying means for supplying liquid rcfrigerant to said evaporator, means for defrosting said evaporator, control means for initiating operation of said defrosting means, said control means including a receptacle in which a fluid migrates from one portion thereof to another portion thereof during operation of said refrigerating system, a thermistor unit disposed in said receptacle at a point therein to have its temperature infincnced by said fluid migration, and means responsive to a change in temperature of said thermistor for initiating operation of said defrosting means.

References Cited in the file of this patent UNITED STATES PATENTS 2,244,892 Newton June 10, 1941 2,302,320 Hintze Nov. 17, 1942 2,459,173 McCloy Jan. 18, 1949 2,679,602 Gallagher May 25, 1954- 2,824,278 Johnston Feb. 18, 1958 

